A method referred to as a spin coating has been widely used in forming a coating film, e.g., a resist film, on a substrate due to its a rapid film forming capability. In the spin coating, a back surface of the substrate is held by a spin chuck, and a resist is supplied to a central portion on a surface of the substrate. The substrate is then rotated such that a centrifugal force causes the resist to expand to a periphery of the substrate. Thereafter, the substrate continues to be rotated to dry the resist on the surface of the substrate, thereby forming the resist film.
In order to rapidly dry the resist, the rotational speed of the substrate may be increased to facilitate volatilization of solvent in the resist. However, an upper limit of the rotational speed is limited by the Reynolds number of a gas stream on the substrate. Specifically, as the rotational speed of the substrate increases, the Reynolds number increases. If the Reynolds number exceeds a certain value, a flow of the gas stream on the substrate becomes turbulent. Then, the turbulent flow is transferred onto a surface of the resist film. That is, concave-convex portions are formed on the resist film along the turbulent flow, thus deteriorating an in-plane uniformity of the resist film in a plane of the substrate.
The size of a semiconductor wafer (hereinafter, referred to as a wafer) used as a substrate has been gradually increased and thus it has been proposed to use a wafer having a diameter of, e.g., 450 mm. If the size of the wafer is increased as described above, the upper limit of the rotational speed that may reduce the formation of the concave-convex portions on the periphery of the wafer is lowered, as represented in evaluation tests to be described below. That is, it takes a long time to dry the resist after the resist is applied, thus resulting in a decrease in throughput.
There is a technology in which a ring-shaped plate is provided above a rectangular substrate so as to face corners of the rectangular substrate such that the gas stream on the substrate with the resist applied thereon as described above is rectified. However, such a plate causes the gas stream moving toward an opening of the plate to be rapidly directed downward near an inner peripheral edge of the plate. When the substrate is subjected to a dry process while being exposed to the gas stream directed downward, a dry speed near the inner peripheral edge of the plate in the plane of the substrate is different from those at other positions. As a result, a thickness of the resist film obtained after the dry process is increased locally near the inner peripheral edge.